Abstract

Al-rich orthopyroxene megacrysts (OPM) are found in three distinct field associations (leuconorite, anorthosite and anorthositic veinlets). The presence of plagioclase inclusions in OPM and local development of subophitic textures require cocrystallization of OPM and plagioclase. OPM have 0.66 < Mg/Mg + Fe T ) < 0.72, within the range of other orthopyroxene types in the massif, but contain higher amounts of Ca, Al, Fe (super 3+) and Ti. REE patterns in OPM and other orthopyroxene are essentially parallel, but OPM are enriched up to a factor of ten. Associated plagioclase-rich materials have An contents, REE patterns, Sr contents and Sr isotopic compositions virtually identical to those found in andesine anorthosite and leuconorite comprising most of the massif. These textural, chemical and isotopic features indicate that the OPM crystallized in the outcrops where they presently reside, from the same anorthositic magma that gave rise to the rocks of the massif as a whole, and that OPM are not relics of remote (upper mantle) crystallization from basalt magma. The incorporation of FeTi and Eskola components in original orthopyroxene may be the result of metastable crystallization, where local overstepping of a plagioclase-orthopyroxene cotectic boundary curve has driven the magma to orthopyroxene supersaturation. The localized appearance of OPM supports this. The higher abundances of Ca, Al, Ti, Fe (super 3+) and REE in OPM compared to other orthopyroxene may be related to the ionic adsorption and retention of these cations onto rapidly growing surfaces of the OPM.--Modified journal abstract.